The present invention relates to multilayer coextruded light-reflecting films which have a narrow reflection band due to light interference. When the reflection band occurs within the range of visible wavelength, the film is iridescent. Similarly, when the reflection band falls outside the range of visible wavelength, the film is either ultraviolet or infrared reflecting. Such multilayer films and methods by which they can be produced are known in the art.
The multilayer films are composed of a plurality of generally parallel layers of transparent thermoplastic resinous material in which the contiguous adjacent layers are of diverse resinous material whose index of refraction differs by at least about 0.03. The film contains at least 10 layers and more usually at least 35 layers and, preferably, at least about 70 layers.
The individual layers of the film are very thin, usually in the range of about 30 to 500 nm, preferably about 50-400 nm, which causes constructive interference in light waves reflected from the many interfaces. Depending on the layer thickness and the refractive index of the polymers, one dominant wavelength band is reflected and the remaining light is transmitted through the film. The reflected wavelength is proportional to the sum of the optical thickness of a pair of layers.
The quantity of the reflected light (reflectance) and the color intensity depend on the difference between the two refractive indices, on the ratio of optical thicknesses of the layers, on the number of layers and on the uniformity of the thickness. If the refractive indices are the same, there is no reflection at all from the interfaces between the layers. In multilayer iridescent films, the refractive indices of contiguous adjacent layers differ by at least 0.03 and preferably by at least 0.06 or more. For first order reflections, reflectance is highest when the optical thicknesses of the layers are equal, although suitably high reflectances can be achieved when the ratio of the two optical thicknesses falls between 5:95 and 95:5. Distinct color reflections are obtained with as few as 10 layers. However, for maximum color intensity it is desired to have been 35 and 1000 or even more layers. High color intensity is associated with a reflection band which is relatively narrow and which has high reflectance at its peak. It should be recognized that although the term "color intensity" has been used here for convenience, the same considerations apply to the invisible reflection in the ultraviolet and infrared ranges.
The multilayer films can be made by a chill-roll casting technique using a conventional single manifold flat film die in combination with a feedblock which collects the melts from each of two or more extruders and arranges them into the desired layer pattern. Feedblocks are described for instance in U.S. Pat. Nos. 3,565,985 and 3,773,882. The feedblocks can be used to form alternating layers of either two components (i.e. ABAB . . . ); three components (e.g. ABCABCA . . . or ACBCACBC . . . ); or more. The very narrow multilayer stream flows through a single manifold flat film die where the layers are simultaneously spread to the width of the die and thinned to the final die exit thickness. The number of layers and their thickness distribution can be changed in inserting a different feedblock module. Usually, the outermost layer or layers on each side of the sheet are thicker than the other layers. This thicker skin may consist of one of the components which makes up the optical core; may be a different polymer which is utilized to impart desirable mechanical, heat sealing, or other properties; or may be a combination of these.
Examination of iridescent films of desirable optical properties revealed deficiencies in certain mechanical properties. For example, the adhesion between individual layers of the multilayer structure may be insufficient, and the film may suffer from internal delamination or separation of layers during use. The iridescent film is often adhered to paper or board for its decorative effect, and is then used for greeting cards, cartons, wrapping paper and the like. Delamination of the film is unsightly and may even lead to separation of the glued joints of carton. In additional, the solvent resistance and heat stability of such films are not as great as desired for widespread utilization.
In U.S. Pat. No. 4,310,584, these deficiencies are significantly overcome by using a thermoplastic terephthalate polyester or copolyester resin as the high refractive index component of the system in which two or more resinous materials form a plurality of layers. While a substantial improvement was realized, it also required the use of two polymers from significantly different polymer families. That fact, in turn, means that there are inherent significant differences between the two polymers and their relative adhesion to each other, chemical resistance, toughness, etc. As a result, the film itself is generally no better than a particular characteristic than the weaker or poorer of the polymers employed. If two polymers closely related were employed in order to maximize relative adhesion to each other, or toughness, or chemical resistance, etc., the polymers involved did not have a sufficient difference in refractive index so as to create the desired iridescent color. It has now been found that further improvements in adhesion, solvent resistance and the like can be obtained by the use of an engineering thermoplastic resin.
Schrenk and Wheatley have reported the preparation of a multilayer light reflecting film co-extruded from two thermoplastic elastomers, Co-extruded Elastomeric Optical Interference Film, Antec '88, 1703-1707. The film, which had one thermoplastic elastomer based on nylon and the other based on a urethane, exhibited reversible changes in reflection spectra when deformed and relaxed. That is, this very specific combination had the ability of stretching without losing appearance characteristics.
Accordingly, it is the object of this invention to provide new and improved multilayer light-reflecting films which exhibit increased resistance to delamination, improved solvent resistance and/or improved heat stability. This and other objects of the invention will become apparent to those skilled in this art from the following detailed description.